1. Field of the Invention
The present invention relates to the field of interface signaling protocol for mixing signals of different media types.
2. Related Art
Capturing a window within a display allows for multiple images to be shown simultaneously. For example, a user may have MPEG-2 video data from one device, such as a MPEG video decoder, being shown on a display. At the same time, the user could be using data from a second device in order to access the internet for graphics data. The video and graphics data are mixed into one data stream going to the display. A window in the display shows the graphics data while simultaneously maintaining the video feed to the entire display.
Synchronization is one particular problem when mixing two streams of data. In the foregoing example, the MPEG video decoder is sending data to the display at certain output timing rates, such as, frame rate, pixel rate, line frequency, etc. Graphics data comes from a source device and is sent to the destination device, the MPEG video decoder. Since the incoming data stream is continuous, in order for the graphics data to be mixed with the video data, the incoming graphics data stream must be synchronized with the video data going to the display to have the same output timing rates. These various timing rates can be associated with various television formats such as Phase Alternating Line (PAL), National Television Standards Committee (NTSC), and High Definition Television (HDTV).
In the past, synchronization has been accomplished by a technique called gen-locking. In gen-locking, a phase lock loop between the two devices locks the output timing of the source device to the output timing of the destination device using the vertical synchronization signals. In this manner, the incoming graphics data can be easily mixed with the video data.
However, gen-locking has posed some problems with jitter in the digital domain. Jitter is an error in the pulse positioning in the digital data stream and shows up as chroma-phase jitter in the analog domain. On the display, jitter shows up as small vibrations or fluctuation in a displayed image. The source of the jitter stems from gen-locking to the digital reference of the destination device. Also, poor-quality gen-locking circuitry can contribute to jitter.
The two types of jitter pertinent to synchronizing between two digital devices are high and low frequency jitter. Problems exist depending on which filter is used to combat the jitter. If a lowpass filter is used in the phase lock loop, you have better signal quality, but the phase lock loop moves with the low frequency jitter and thus the low-frequency jitter is allowed to pass giving a jittery picture. Additionally, synchronization can be easily lost.
On the other hand, if a highpass filter is used to filter the signal, the high-frequency jitter can be removed through various techniques, such as reclocking the signal. However, the signal quality is reduced.
Furthermore, another technique to synchronize two continuous streams of data is to use a large buffer, or multiple buffers, for holding the incoming stream data, such as the graphics data in the above example. The incoming graphics data is written into the memory buffer at one rate, the output rate of the source device, and read out of the buffer at the same rate as the destination device. However, the main disadvantage of this scheme is that it can require a significant amount of memory, at least two entire pictures.
Additionally, with the introduction of high definition digital television (HDTV) the buffer size must be increased. Increasing buffer size to synchronize the incoming data stream reduces the bandwidth available to the display, especially if there are multiple windows showing in the display. The result is that the required memory bandwidth is increased to prevent underrunning the display.
Thus, a need exists for a more efficient method for mixing signals of different media types. Additionally, a need exists for improving bandwidth management when interfacing signals of different media types.